Plant oils represent a renewable resource of highly reduced carbon. Current world vegetable oil production is estimated at 87 million metric tons with an approximate market value of some 40 billion U.S. dollars. The majority of vegetable oil currently goes directly to human consumption and as much as 25% of human caloric intake in developed countries is derived from plant fatty acids (Broun et al. (1999) Ann. Rev. Nutr. 19: 197-216). In addition to their importance in human nutrition, plant fatty acids are also major ingredients of nonfood products such as soaps, detergents, lubricants, biofuels, cosmetics, and paints (see Ohlrogge (1994) Plant Physiol. 104: 821-6). While the demand for vegetable oils has increased steadily, production capacity to meet this demand is more than adequate and prices of vegetable oils have remained below or near 0.6 U.S. dollars per kilogram. This low cost of production has stimulated interest in use of vegetable oils as renewable alternatives to petroleum-derived chemical feedstocks.
Fatty acids are the most abundant form of reduced carbon chains available from nature and have diverse uses ranging from food to industrial feedstocks. Plants represent a significant renewable source of fatty acids because many species accumulate them in the form of triacylglycerol as major storage components in seeds. With the advent of plant transformation technology, metabolic engineering of oilseed fatty acids has become possible and transgenic plant oils represent some of the first successes in design of modified plant products. For example, the transfer of a California bay plant thioesterase gene into the seeds on non-laurate (12:0)-accumulating plants, Arabidopsis and Brassica napus (rapeseed) resulted in the alteration of the fatty acid acyl chain elongation process to produce laurate up to 24% and 58% of total seed fatty acids, respectively (see Voelker et al. (1992) Sci. 257: 72-4; and Voelker et al. (1996) Plant J. 9: 229-41 respectively). Thus, the transfer of a single gene into a plant can dramatically alter the type of fatty acids produced.
However, such success with a single gene is the exception rather than the rule (for review, see Thelen and Ohlrogge (2002) Metabol. Engineer. 4:12-21). Moreover, to be economically useful for both human consumption and industrial uses, an actual increase in seed oil fatty acid content, rather than just a change in the type of fatty acid produced, would be highly desirable. While the production of malonlyl-CoA by acetyl-CoA carboxylase is a key regulatory step in the de novo synthesis of fatty acids, attempts to increase the rate of this apparently rate-limiting step by genetic engineering have met with, at best, modest success. Furthermore, the overexpression of several individual fatty acid synthase enzymes has not resulted in an increased flux of fatty acid biosynthesis (reviewed in Thelan and Ohlrogge (2002) Metabol. Engineer. 4:12-21).
Accordingly, there remains a need for genetic engineering strategies that will increase the total amount of fatty acid produced by plants.